This invention relates to a vibration absorbing device for reducing or eliminating vibrations in bodies that are subjected to periodic or oscillating stress. More specifically, it is based on the realization that mechanical energy, the source of all vibrations, can be transmitted in the form of periodic stress through a mechanical continuum comprised of two or more elastic solids.
The present invention therefore teaches that periodic stress can be transmitted directly from one elastic body, in which the periodic stress first originated, into a second elastic body comprising a vibration absorbing damping plate or a plurality of such damping plates. In practice, these plates may either be selectively tuned to specific frequencies or they may be tuned in a manner that causes their frequencies of natural resonance to vary continuously over a selected broad wavelength band of vibrations. Consequently, the damping plates will resonate sympathetically in response to any periodic stress that may be transmitted into them from a vibrating body. During such a continuous energy transmission process, the damping plates are excited into sympathetic resonance, causing mechanical energy, the source of all vibration, to be dissipated, not as much in the body in which this energy first originated, but more so in the vibrating modes of the damping plates comprised in the vibration absorbing device of this invention.
Such a vibration absorbing device is capable of reducing or eliminating vibrations conducted in solids, particularly vibrations which emanate from loudspeaker enclosure panels and wall elements whereby the vibration absorbing device, when mounted directly to the loudspeaker enclosure or wall, absorbs vibrations, not by exerting opposing or restraining forces on a vibrating body as a means of attenuating the vibrations, but by functioning as an independently tuned, mass-spring system. It may contain a single vibration absorbing damping plate or a plurality of such damping plates, the cross-sections of which are shaped and affixed to spacer and mounting means in such a manner that their natural resonances are either tuned to specific frequencies or to continuously varying frequencies within a selected broad wavelength band or bands of vibrations.
Vibrations that are produced in elastic solids represent the recovery of mechanical energy that has been stored in the body. Accordingly, in loudspeaker enclosures, the loudspeaker drivers, which are mounted to the enclosure, transmit mechanical energy into the enclosure, causing the enclosure panels to vibrate. The problem that arises with this is that the panel radiated sound waves resulting from these panel vibrations will not be synchronized with the loudspeaker's direct sound waves. Instead, these sound waves will be somewhat delayed behind the loudspeaker's direct sound waves, thereby causing phase and amplitude errors in the sound radiation pattern of the loudspeaker. These response errors will therefore cause a listener to perceive sounds emanating from the loudspeaker as being distorted, thereby causing listening fatigue. In addition, the periodic stress that is transmitted into walls by sound waves emanating from a loudspeaker or other energy source will cause the wall to vibrate. The resulting sound waves that are radiated back into a room by these wall vibrations are known to be destructive and will therefore have a deleterious acoustical effect upon a listener. Thus, eliminating vibrations from loudspeaker enclosures and walls, and the deleterious acoustical effect that these vibrations have upon listeners, would be a major step in achieving flawless fidelity in the reproduction of sound.
Heretofore, a wide variety of vibration damping methods have been proposed or implemented to reduce or eliminate vibrations and their deleterious effect upon listeners. For example, in loudspeaker enclosures, rigid braces are often used as a means of offering resistance against the inside walls of the enclosure panels and thus reduce the panel vibrations. However, in practice, such bracing techniques will have little effect in reducing panel vibrations at those locations in the enclosure where no such bracing is employed. The enclosure, with its remaining panels free to oscillate sympathetically in response to the periodic stress propagating through them will therefore continue to vibrate and radiate delayed energy sound waves into the adjacent air.
Still other vibration absorbing devices have been proposed or implemented to attenuate vibrations in elastic solids. Two such devices, one of which is described in U.S. Pat. No. 2,270,902 to Rubissow and the other in U.S. Pat. No. 4,392,681 to Raquet, are comprised of a heavy weight or a plurality of such weights which, due to their inert mass, are disposed to exert opposing forces upon a vibrating body by means of a rubbery or spongy intermediate member that is layered between the heavy weight and the vibrating body. As a result, the vibrations are attenuated because a portion of the mechanical energy causing the body to vibrate has been dissipated through compression of the rubbery intermediate member residing between the heavy weight and the vibrating body. However, these types of vibration absorbing devices, by relying on opposing forces which are made available by an inert mass to retard the velocity of a vibrating body and thus reduce the amplitude of the vibrations, are limited in their ability to absorb vibrations from certain vibrating bodies, particularly from loudspeaker enclosures and walls. This is because, unlike the present invention, these devices are not comprised of bodies that are freely suspended and specifically tuned to receive by way of direct transmission, and to recover from the entire vibrating body by way of sympathetic resonance, a major portion of the periodic stress propagating throughout the entire body. Consequently, a major portion of this periodic stress will continue to be recovered as vibrations at those locations in the loudspeaker enclosure or wall where neither of the vibration absorbing devices described in U.S. Pat. Nos. 2,270,902 and 4,392,681 has been attached to oppose the periodic forces and attenuate the vibrations.
Another device and method that has been implemented to attenuate vibrations in a vibrating body is a vibration absorbing assembly developed by the application in this invention and described in U.S. Pat. No. 5,240,221. However, this device differs from the present invention in the following respects:
It is well known that the periodic or oscillating stress that loudspeaker enclosures are subjected to during a loudspeaker's operation will propagate at high velocity throughout the entire enclosure. Since the values in mass and compliance which govern the natural resonance frequencies of the individual enclosure panels and portions of these panels are vast in number, it can be expected that most loudspeaker enclosures and walls will vibrate over a vast number of different frequencies, all of which reside in the audible sound spectrum. In view of the foregoing, it will be seen that the damping plates comprised in the vibration absorbing assembly described in U.S. Pat. No. 5,240,221 are shaped in their cross-sectional area and affixed to spacer and mounting means in a manner that limits their natural resonance modes to a relatively small number of specific frequencies. Consequently, these damping plates will not be able to produce a wide range of natural resonances that continuously vary in sympathetic response to vibrations that, in loudspeakers and walls, are know to vary continuously throughout a broad range of frequencies residing in the audible sound spectrum.
In view of the above, it is the aim of the invention to achieve the following objects, singly or in combination:
To provide a vibration absorbing device comprised of damping plates that are composed of imperfectly elastic materials, whereby the damping plates may either be selectively tuned to one or more specific frequencies or they may be variably tuned to resonate sympathetically with the continuously varying periodic stress generated in the vibrating body to which the vibration absorbing device is connected and, thereby, since the damping plates are composed of materials that are imperfectly elastic in nature, recover such periodic stress in the bending modes of the damping plates as heat.
To connect the vibration absorbing device to a vibrating body and thus form a mechanical continuum comprised of the vibrating body and the vibration absorbing device wherein the specifically tuned or variably tuned damping plates comprised in the vibration absorbing device are able, by reason of their imperfectly elastic material composition, tuning and lower inert mass, to recover as heat the periodic stress propagating through the continuum.
To connect to a loudspeaker enclosure panel a variably tuned damping plate or a plurality of such plates wherein the single plate or the plurality of such plates are provided a total surface area approximating that of the speaker enclosure, with mounting means shaped in such a way and having such area so as to keep the damping plates continuously coupled to the enclosure panel across the entire length of the panel to which they are connected and, in so doing, provide the opportunity for maximum transfer of mechanical energy from the enclosure into the variably tuned damping plates, thus causing a major portion of this energy to be dissipated in the damping plates, thereby making this energy unavailable in the enclosure to produce panel vibrations and their destructive audible effects.
To connect the vibration absorbing device with a vibrating body and thus add to the vibrating body a second body whose tuned damping plates have a combined mass that is less than that of the vibrating body thereby causing the damping plates, by reason of their specific tuning and lower inert mass, to be better suited for recovering mechanical energy that may be transmitted into them from the vibrating body than the vibrating body is itself.
To improve a vibration absorbing device in such a manner that, in contrast to prior art, it does not exert opposing forces either into itself or onto a vibrating body as a means of retarding the velocity of a vibrating body and thus reduce the amplitude of the vibrations.
To provide in the vibration absorbing device one or more damping plates stacked together, wherein each damping plate has a different cross-sectional area than the others, and by means of spacers to separate the plates from each other, to construct these spacers so that their opposite sides, which are in contact with adjoining plates, are offset in opposite directions from each other but at equal distances from the centers of the adjoining plates; and thus, through the combination of continuously varying plate sizes and offset spacers, to provide continuously varying values in mass and compliance that cause the damping plates in the vibration absorbing device to exhibit natural resonance modes that continuously vary in frequency over a selected broad wavelength band or bands of vibrations.
To provide a vibration absorbing device wherein the selectively tuned damping plates comprised in the device are supported by means of spacers which are positioned and affixed between the damping plates, thereby enabling the plates to vibrate freely in space in accordance with the mass and compliance factors which govern their natural resonance frequencies whereby the spacers, separating each damping plate from the adjoining plates, form a discrete transmission path for the direct transmission of mechanical energy from a vibrating body into all of the damping plates, thereby allowing periodic stress to excite the damping plates into sympathetic resonance giving them complete freedom to oscillate in free air, unimpeded in their motion by any other internal or external forces acting upon them.